Electrolytic means for detecting and measuring the pH (a measurement of the hydrogen-ion activity of a liquid system) are well known. Generally such pH sensors include a glass membrane type electrode and a reference electrode. These glass electrodes tend to be quite fragile, and are therefore not generally suitable for applications involving a considerable amount of movement or jostling or shock. In addition these glass membrane type electrodes generally manifest an elevated electrical impedance. This elevated electrical impedance can make electrical circuitry necessary for evaluating an electrical signal produced by the glass electrode and providing information as to the pH of a solution in which the glass electrode is immersed, more complicated, and often more expensive.
Other pH sensing electrodes, for example those utilizing a polymeric electroactive material, have been proposed. Such polymeric based electrodes, installed subcutaneously, have been utilized to measure, for example, the pH within a human body. One difficulty with polymeric electrodes generally concerns selectivity, particularly with respect to interference arising from the presence of potassium or sodium ions both present in significant quantity in the human body.
pH electrodes of a junction type comprising a palladium wire and including palladium oxide have been proposed for sensing the pH of solutions and other fluids. In addition to an active or pH electrode, a reference electrode is generally required where measuring pH. A wire-like palladium/palladium oxide junction type pH electrode can complicate the making, positioning and supporting of a suitable reference electrode, particularly where the pH electrode and reference electrodes are being miniaturized. Junction type electrodes include an interface or transition region between an electrical conductor and an electrical semi-conductor.
It has long been known that in fluids and fluid solutions otherwise identical but for differing quantities of dissolved carbon dioxide, the pH of each respective solution may be closely correlated to the carbon dioxide content of the solutions. Therefore, sensing devices having electrodes useful for the determination of a pH, often may find utility in determining the dissolved carbon dioxide content of a particular fluid.
It has been proposed that pH sensors be employed for the non-invasive measurement of blood carbon dioxide in humans and animals. In such blood carbon dioxide sensors, a pH electrode and a reference electrode generally are bundled together, encapsulated in a selectively permeable polymeric membrane. The polymeric membrane generally includes an electrolyte, the membrane being impermeable to the elecrolyte but permeable to carbon dioxide gases. The bundle, including probe, reference probe, and encapsulating membrane are applied to a human or animal skin surface. Carbon dioxide diffusing from the body of the human or animal through its skin surface passes through the selective membrane becoming dissolved in the electrolyte solution. The pH of the electrolyte solution is thereby altered, an alteration reflected in the electrical output from the pH active electrode. As the electrical output of reference electrode typically is not effected by such changes, a measurement of changes in the electrical output between the reference and active electrodes provides an indication of changes in the value of the pH. Such transcutaneous, or across the skin, blood carbon dioxide gas sensors are often referred to as a type of Severinghaus electrodes.
With Severinghaus electrodes, two additional difficulties are often encountered in supplement to those difficulties common to pH glass electrodes. Such Severinghaus electrodes generally require a heating of the human or animal skin to which they are applied to a range of temperature of between 43.degree. C. and 45.degree. C. so as to promote vasodilation of subcutaneous blood vessels adjacent the sensor. Vasodilation promotes blood flow through regions of the skin adjacent the sensor and thereby increases the amount of carbon dioxide available for diffusion through the skin and therefore available for sensing at the Severinghaus electrode. These elevated temperatures may contribute to a burning of the skin to which such a Severinghaus electrode is applied. Particularly, a temperature of 45.degree. C. can often produce a quite significant skin burning effect in a relatively short period of time.
A second problem relates to a requirement that, particularly, glass electrodes comprising a Severinghaus electrode be immersed in electrolyte. Such an immersion of necessity requires that the membrane retain a pool of electrolyte in contact with the Severinghaus electrode. Minor imperfections, defects, or wounds to the membrane can necessitate replacement of the membrane, an exacting procedure often requiring specialized equipment. Following a membrane change-out, generally it is necessary that the resilting rebuilt Severinghaus electrode be recalibrated. Recalibration can also require somewhat specialized equipment and can be time consuming.
Typical electrode heating assemblies are shown in U.S. Pat. No. 4,290,431 (Herber) wherein a semiconducting type heating device is utilized within a transcutaneous oxygen sensor. Similarly, U.S. Pat. No. 4,296,752 (Welsh) provides a heater adjacent the electrode assembly incorporated within the main body of a transcutaneous sensor. These patents, U.S. Pat. No. 4,259,963 (Huch) and British Pat. No. 2,056,689A (Heist) provide examples of a typical membrane configuration.
U.S. Pat. No. 4,276,144 (Hahn) proposes the use of a polymeric gas permeable layer over the end of an electrode in a multi-electrode assembly but fails to show how such materials may be employed in a transcutaneous blood gas sensor assembly. An example of an apparatus for oxygen partial pressure measurement incorporating a transcutaneous blood oxygen sensor is shown, for example, in U.S. Pat. No. 4,269,684 (Zick). Zick, however, suggests a standard type of electrode and liquid electrolyte reservoir.
An extended life, preferably solid state, low electrical impedance, pH sensor including an active electrode and a reference electrode both having an extended service life could find substantial utility in pH measurement functions. Where such a sensor can be encapsulated in a carbon dioxide permeable, electrolyte nonpermeable, polymeric membrane, such a pH sensor offers potential utility for use as a transcutaneous blood carbon dioxide sensor. Particularly where such a blood carbon dioxide gas sensor includes active and reference electrodes that are replaceable without requiring recalibration of such a sensor, its utility, particularly in the field of medical products, is quite promising.